AbstractCycloartenol (4), a natural plant sterol, was shown to be an effective membrane reinforcer; this was attributed to its conformation. We now present a conformational analysis of4by molecular modeling and NMR. Molecular modeling suggests that two conformationsIandIIcoexist, differing mainly at the level of ring C, and of nearly equal energy,IandIIeach having ring A and B in a chair and half‐chair conformation, respectively, with ring C 1,3‐diplanar inI(solid‐state structure as determined by X‐ray crystallography) and in chair conformation inII. A complete assignment of the1H‐ and13C‐NMR spectra of4and the entire coupling network in rings A and B is determined by various modern NMR techniques. The conformation of rings A and B thus determined is in agreement with conformationsIandII. Low‐temperature NMR experiments show a fast equilibrium between two conformations, presumablyIandII. It is concluded, therefore, that the cyclopropane ring of4produces a flexibility at the level of ring C which may be important for the membra